But the best way to have Fin Fang Foom in an Iron Man movie is with Mandarin

If the rumor is true and that levithan in The Avengers movie indeed turns out to be Foom, then it will be introduced without the presence of Mandarin. Personally, I don't see how these two need to be together anyway, and they can always rewrite the origin of Mandarin's ten rings to fit the MCU. You should know by now that movies are not required to be consistent with their comics counterpart.

__________________"I know I'm asking a lot, but the price of freedom is high, it always has been, and it's a price I'm willing to pay. And if I'm the only one, then so be it. But I'm willing to bet I'm not." - Captain America